The migration is not an event with a date. It is a continuous, year-round movement of roughly 1.5 million wildebeest with several hundred thousand zebra and gazelle, circling the Serengeti–Mara ecosystem in pursuit of new grass. It is always happening; the only question is where.
Roughly: December to March the herds are on the short-grass plains of the southern Serengeti and Ndutu, where the calving happens. April and May they begin moving north-west through the central Serengeti with the long rains. June and July they push into the Western Corridor and face the Grumeti River. August and September they are in the far north around the Mara River, where the famous crossings happen. October and November they turn and come back south through Lobo and Loliondo.
The river crossings people picture are an August-to-September phenomenon in the north, and they cannot be scheduled — herds may gather at the bank for two days and then not cross, or cross three times in one morning. If a crossing is the reason for your trip, budget at least three nights in the north to give yourself the chances. Whatever the month, we place your camps where the herds are expected to be, and mobile camps move with them.